This application is based on Patent Application Nos. 312869/1997 filed on Oct. 30, 1997 in Japan, 361461/1997 filed on Dec. 26, 1997 in Japan and 361462/1997 filed on Dec. 26, 1997 in Japan, the content of which is incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to an ink, especially an ink used in the process of ink jet printing and to an ink jet printing method. The present invention also relates to an ink container, an ink set, and an ink jet printing apparatus. The present invention can be applied to all of the devices such as printers, facsimile machines, and copying machines, which are able to print on various kinds of printing mediums such as paper, fabric, leather, non-woven fabric, and plastic sheets (e.g., OHP sheet).
2. Description of the Related Art
An ink jet printing method has been used in a broad range of printers, copying machines, and so on because of its various advantages including: low noise generation, low running cost, high-speed printing, size reduction, and output in multiple colors. Each of such printing and copying machines makes a choice among different types of inks generally in terms of: ink-discharging properties, printing properties including fixation of ink, bleeding and reflection density of print image, and print qualities including color development.
By the way, it is well known that those inks can be broadly divided into two groups, i.e., dye inks and pigment inks in terms of coloring materials contained in the respective inks. The pigment inks are superior to the dye inks in water resistance, light stability, and ability to provide excellent character qualities such as sharp outline of characters.
One of the examples of pigment inks that contain a pigment dispersant (hereinafter, referred as a dispersant-containing pigment ink) is disclosed in, for example, Japanese Patent Application Laid-open Nos. 179183/1993 and 53841/1995. The former discloses a pigment ink in which a pigment is dispersed by a block polymer of xe2x80x9cABxe2x80x9d or xe2x80x9cBABxe2x80x9d type, and the latter discloses another pigment ink in which a pigment is dispersed by a tri-block polymer of xe2x80x9cABCxe2x80x9d type.
Furthermore, a self-dispersing type pigment ink (hereinafter, referred as a dispersant-less pigment ink) which does not need to use any dispersants such as the block polymer described above to disperse the pigment is also known. For example, International patent application Nos. WO96/18695 and WO96/18696, disclose pigment inks in which carbon blacks to whose surfaces hydrophilic groups are directly bound, are dispersed.
In case of applying either types of the pigment inks, i.e. the dispersant-containing pigment ink and the dispersant-less pigment ink, onto a printing medium, pigments in the ink are agglomerated on a surface of the printing medium, and form an image dot. Thus, the coloring material, i.e. pigments, are put on the surface of the printing medium. That is to say, almost all the coloring materials in the ink tend to remained on the surface of the printing medium. Hereinafter, ink providing this kind of image dot, is called xe2x80x9ctop-loading typexe2x80x9d ink.
On the other hand, in order to improve the printed image""s color characteristics, the present inventors made an invention in which so called super-penetration type ink, i.e. an ink which penetrates into the inside of a printing medium extremely well, is employed. In the invention, the super-penetration type ink""s penetration into the printing medium in its thickness direction is accelerated and the super-penetration type ink is allowed to spread in the printing medium (see European Patent Application No. EP 583096 A, or Japanese Patent Application Laid-open No. 88048/1994).
In view of the aforementioned prior art, the present inventors focused on the dot shape including the dot density""s uniformity, the outer shape of the dot, and the diameter of the dot etc., and have obtained the following findings. FIGS. 7A to 7G are schematic plain views of image dots obtained by applying various types of inks to the same printing medium under same conditions.
FIG. 7A is an image dot having a diameter of RA formed with a dispersant containing ink, and FIG. 7B is an image dot having a diameter of RB formed with a dispersant-less ink. The diameter RB was slightly larger than RA. Each of the image dots had unified density throughout the respective image dots. Further, substantial feathering could not be seen in both the image dots, thus any problems could not be found in the outer shapes of the respective dots. Regarding the image dot""s diameter, however, the inventors found the fact that the respective image dots""diameters were slightly smaller than those expected from a shape of respective ink droplets. The reason is believed to be that the image formation by the pigment ink owes to pigments"" agglomeration on the printing medium. To sum up, the pigment ink provides an image dot whose density is unified and has a good outer shape, but the ink droplet seems not to be spread easily on the printing medium. Therefore, regarding ink jet recording process with the pigment ink, the present inventors recognized that the known pigment ink should be modified in order to form an image dot having larger area-factor, i.e. a ratio of an area covered with an image dot to the unit area.
Based on the aforementioned recognition, the inventors modified the conventional pigment inks so as to have better penetration by utilizing the technology of the super-penetration type ink. As a result of the experiment, a change in pigments agglomeration could be observed, and the outer shape of an image dot was changed. As shown in FIGS. 7C and 7D, the diameter of RC and RD of the respective image dots became larger than the diameter of RA and RB. That is, the relation between RA, RB, RC and RD are as follows: RA less than RB less than RC less than RD.
As shown in FIG. 7C, however, a modified dispersant-containing pigment ink whose penetrability was better than that of the conventional dispersant-containing pigment ink, provided an image dot having feathering 701 extending from the center of the image dot towards the outside thereof. In addition, a modified dispersant-less pigment ink having better penetrability than that of the conventional dispersant-less pigment ink, provided an image dot having a circumferential area (hazy area) 703 around the core area 704 in addition to feathering 701. In the circumferential area 703, fine particles of the pigment in the ink were diffused and the density of the circumferential area 703 became lower than that in the core area 704. That is, the density in the image dot was apparently different in its circumferential area 703 and its core area 704.
In accordance with the findings shown in FIGS. 7C and 7D, the present inventors conducted a further experiment regarding image dots formed with two types of further modified pigment inks, one of which was a dispersant-containing pigment ink containing a dye and having better penetrability than that of the conventional dispersant-containing ink, and the other was a dispersant-less pigment ink containing a dye and having better penetrability than that of the conventional dispersant-less pigment ink. The resultant image dots with the respective further modified pigment inks are shown in FIGS. 7E and 7F. The respective image dots have diameters of RE and RF both of which were much larger than RAxe2x88x92RD, and RF is larger than RE. As shown in FIG. 7E, however, the ink dot had a circumferential area 705 around a core area 706, the circumferential area 705 having lower density than that of the core area 706, which was not observed in the image dot shown in FIG. 7C. The circumferential area 705 seems to be made due to separation of the dye and the pigment on the printing medium, and the separated dye""s diffusion, and the core area seems to contain agglomerated pigment. Thus, the difference in density in the image dot became conspicuous.
Furthermore, as shown in FIG. 7F, the ink dot had a circumferential area 707 in which the dye was mixed and diffused in the circumferential area 703 of the image dot shown in FIG. 7D.
Apart from the aforementioned issue regarding the dot shape, the present inventors also found that the conventional pigment ink gave an image lack of uniformity in the pigment fixation resulting from an acute agglomeration of the pigment on the printing medium when the printing medium had poor ink absorbency.
Further, in the case that the strength of the acute agglomeration was relatively strong, not only the non-uniform agglomeration of the pigment but also cracks where no coloring material existed were sometimes observed in the image dot.
FIG. 1 is a schematic view for explaining the cracks. As one can see therefrom, the size of the crack is relatively large and conspicuous. Thus the cracks make the quality of the printed image get worse. In addition to that, the cracks may decrease optical density (O.D.) of the image dot since the surface of the printing medium is exposed at the cracks. This kind of crack is observed when the conventional pigment ink is applied to a printing medium having a top-coat layer for receiving the ink, i.e. a resin layer, like a transparency for over head projector (OHP) etc., because the agglomeration of the pigment on the resin is affected by materials in the resin layer. In particular, in the case of applying an anionic pigment ink to the printing member provided with a resin layer containing a cationic substance, the pigment agglomeration occurs drastically on the resin layer.
The issue about the agglomeration which causes the cracks is conspicuous when the conventional pigment ink is employed, but the present inventors confirmed that the cracks occurred in an image dot formed with an ink containing a dye, a pigment and a dispersant for the pigment disclosed in Japanese Patent Application Laid-open No. 276873/1990 which was for by the present applicant.
The present invention is completed in view of the technical problems for providing the more excellent quality of image formed by the use of an ink-jet printing method, which are additional problems being found as a result of careful consideration on the related art as described above.
An object of the present invention is to provide an ink capable of providing an excellent image dot in its shape, i.e. an ink dot spread on a printing medium properly and having an appropriate diameter in terms of an ink droplet applied to the printing medium, having uniform density in the dot, being free from or substantially free from any feathering in the circumferential area, and having a good outer shape.
Another object of the present invention is to provide an ink capable of reducing unevenness of a printed image due to pigment""s agglomeration, and deterioration of a printed image""s quality caused by cracks.
A further object of the present invention is to provide an ink jet printing process capable of forming a high quality image with enough density and reducing quantity of an ink applied to a printing medium.
Still another object of the present invention is to provide an ink jet printing process capable of forming a high quality image with no cracks or substantially no cracks on a printing medium having poor ink absorbency.
A still further object of the present invention is to provide an ink jet recording apparatus capable of recording a high quality image on a printing medium stably.
A still further object of the present invention is to provide an ink cartridge and an ink tank for an ink jet recording apparatus capable of recording a high quality image.
According to a first aspect of the present invention, there is provided an ink containing an anionic dye and a self-dispersant type pigment wherein an anionic group is bonded to a surface of the pigment, the anionic group being bound to the pigment""s surface directly or through an atomic group, wherein the ink has Ka value of less than 1 mlxc2x7mxe2x88x922xc2x7msecxe2x88x92xc2xd according to the Bristow method.
According to another aspect of the present invention, there is provided an ink for use in an ink jet image forming process, the ink being ejected from an orifice by an ink jet ejecting process, comprising an anionic dye and a self dispersant type pigment wherein an anionic group is bonded to a surface of the pigment, the anionic group being bound to the pigment""s surface directly or through an atomic group, wherein the ink has a Ka value of less than 1 mlxc2x7mxe2x88x922xc2x7msecxe2x88x92xc2xd according to the Bristow method.
The respective inks can provide image dots having larger diameters than those of dots formed with conventional xe2x80x9ctop-loading typexe2x80x9d pigment inks, i.e. a dispersant-containing pigment ink and a dispersant-less pigment ink. As shown in FIG. 7G, the relation among the dot diameter RG, RA and RB was as follows: RA less than RB less than RG. Using the present inks, it is possible to form an image having enough density on a certain printing medium which tends to be difficult to have a high quality image formed thereon without increasing the quantity of the ink to be applied. In addition, when compared with an ink dot formed with a modified conventional pigment ink having better penetrability, the present image dot tends to be smaller than the diameter RD. However, the present dot has more uniform density and higher optical density, and feathering cannot be seen around the dot. That is, as shown in FIG. 7G, a hazy area 703 in FIG. 7D or unevenness in image density between a core area and a circumferential area admitted in FIGS. 7E and 7F cannot be seen in the present dot.
The reason why the present inks accomplish these technical advantages is not clear at present, but it is believed that the dye in the present inks interferes with the agglomeration of the self-dispersant type pigment, and the interference prevents the self-dispersant type pigment from agglomerating too much, but allows the self-dispersant type pigment""s agglomeration, and the dye and the pigment are distributed uniformly as if the dye and the pigment unified as one body to form the image dot.
Therefore, regarding dot diameter, the present ink provides image dots having larger diameter than those of dots formed with inks containing only pigments as shown in FIGS. 7A and 7B, and regarding image density, the present ink provides an image dot having highly uniform density and no hazy area because of restriction of spreading fine particles in the ink, such as a dye and a pigment in comparison with inks having better penetration as shown in FIGS. 7C and 7D.
Whatever the mechanism of the present invention, in the case that the present ink containing a self-dispersant type pigment and dye is employed for an ink jet printing, the pigment agglomeration becomes weaker, and the dye moderates the pigment""s agglomeration, thus the pigment""s agglomerated form seems to become a fine particle. In addition, the fine particle of the agglomerated pigment is surrounded by the dye. Then in the resulting printed image, unevenness due to the strong pigment""s agglomeration is resolved.
According to another aspect of the present invention, there is provided an ink-jet printing process comprising the steps of:
i) ejecting an ink by ink jet recording process; and
ii) applying the ink on a recording medium, wherein the ink contains an anionic dye and a self-dispersant type pigment wherein an anionic group is bonded to a surface of the pigment, the anionic group being bound to the pigment""s surface directly or through an atomic group, and
wherein the ink has a Ka value of less than 1 mlxc2x7mxe2x88x922xc2x7msecxe2x88x92xc2xd according to the Bristow method.
According to a further aspect of the present invention, there is provided an ink jet printing process comprising the steps of:
i) ejecting the ink towards a coated layer constituting an outer surface of a recording medium by a ink jet ejecting process; and
ii) forming an image on the outer surface of the recording medium,
wherein the ink contains an anionic dye and a self-dispersant type pigment wherein an anionic group is bonded to a surface of the pigment, the anionic group being bound to the pigment""s surface directly or through an atomic group, and
wherein the ink has a Ka value of less than 1 mlxc2x7mxe2x88x922xc2x7msecxe2x88x92xc2xd according to the Bristow method.
According to each of the above aspects of the present ink jet printing processes, it is possible to form an image dot having a larger diameter than those of image dots formed with conventional xe2x80x9ctop-loading typexe2x80x9d pigment inks, thus it is possible to increase the area factor and to form an image dot having much higher density without increasing the quantity of an ink on a printing medium. Further, the present process provides an image dot having extremely even density compared to dots formed with modified inks which the present inventors studied, i.e. a super-penetration type pigment ink and a super-penetration type dye ink. Therefore, much higher quality images can be obtained.
Furthermore, according to each aspect of the present ink jet printing process, even if an image dot is formed on a printing medium whose ink absorbency is poor, cracks cannot be seen in the dot, and density thereof is high. The reason why such an advantage can be obtained is not clear at present. As mentioned above, it is believed that the degree of the pigment""s agglomeration in an ink containing self-dispersant type pigment and not containing any dispersants such as a polymer, is relatively weak, and the dye in the present ink interferes with the self-dispersant type pigment""s agglomeration, thus it is difficult for the pigment to form a relatively large size of agglomerated pigment on the printing medium. As the result of that, it is possible to reduce the occurrence of the cracks.
According to a still further aspect of the present invention, there is provided an ink tank containing an ink, wherein the ink contains an anionic dye and a self-dispersant type pigment wherein an anionic group is bonded to a surface of the pigment, the anionic group being bound to the pigment""s surface directly or through an atomic group, and wherein the ink has a Ka value of less than 1 mlxc2x7mxe2x88x922xc2x7msecxe2x88x92xc2xd according to the Bristow method.
According to a still further aspect of the present invention, there is provided an ink set comprising a black ink, and at least one color ink selected from a yellow ink, a magenta ink and a cyan ink, wherein the black ink contains an anionic dye and a self-dispersant type carbon black wherein an anionic group is bonded to a surface of the carbon black, the anionic group being bound to the carbon black""s surface directly or through an atomic group, and wherein the ink has a Ka value of less than 1 mlxc2x7mxe2x88x922xc2x7msecxe2x88x92xc2xd according to the Bristow method.
According to a still further aspect of the present invention, there is provided an ink jet recording apparatus comprising four ink containers, each of which contains a black ink, an yellow ink, a magenta ink or a cyan ink, and means for ejecting the respective inks independently by an ink jet ejecting process, wherein the black ink contains an anionic dye and a self-dispersant type carbon black wherein an anionic group is bonded to a surface of the carbon black, the anionic group being bound to the carbon black""s surface directly or through an atomic group, and wherein the ink has a Ka value of less than 1 mlxc2x7mxe2x88x922xc2x7msecxe2x88x92xc2xd according to the Bristow method.
According to these aspects of the present invention, a much higher quality ink-jetted printed image in comparison with the conventional art may be provided.
The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings.